The proton exchange membrane or polymer electrolyte membrane fuel cell (PEM-FC) has been a topic of highly active R&D efforts during the past two decades. The operation of a fuel cell normally requires the presence of an electrolyte and two electrodes, each comprising a certain amount of catalysts, hereinafter referred to as electro-catalysts. A hydrogen-oxygen PEM-FC uses hydrogen or hydrogen-rich reformed gases as the fuel while a direct-methanol fuel cell (DMFC) uses methanol solution as the fuel. The PEM-FC and DMFC, or other direct organic fuel cells, are collectively referred to as the PEM-type fuel cell.
A PEM-type fuel cell is typically composed of a seven-layered structure, including a central polymer electrolyte membrane for proton transport, two electro-catalyst layers on the two opposite sides of the electrolyte membrane in which chemical reactions occur, two gas diffusion layers (GDLs) or electrode-backing layers stacked on the corresponding electro-catalyst layers, and two flow field plates stacked on the GDLs. Each GDL normally comprises a sheet of porous carbon paper or cloth through which reactants and reaction products diffuse in and out of the cell. The flow field plates, also commonly referred to as bipolar plates, are typically made of carbon, metal, or composite graphite fiber plates. The bipolar plates also serve as current collectors. Gas-guiding channels are defined on a surface of a GDL facing a flow field plate, or on a flow field plate surface facing a GDL. Reactants and reaction products (e.g., water) are guided to flow into or out of the cell through the flow field plates. The configuration mentioned above forms a basic fuel cell unit. Conventionally, a fuel cell stack comprises a number of basic fuel cell units that are electrically connected in series to provide a desired output voltage. If desired, cooling and humidifying means may be added to assist in the operation of a fuel cell stack.
Several of the above-described seven layers may be integrated into a compact assembly, e.g., the membrane-electrode assembly (MEA). The MEA typically includes a selectively permeable polymer electrolyte membrane bonded between two electrodes (an anode and a cathode). A commonly used PEM is poly(perfluoro sulfonic acid) (e.g., Nafion® from du Pont Co.), its derivative, copolymer, or mixture. Each electrode typically comprises a catalyst backing layer (e.g., carbon paper) and an electro-catalyst layer disposed between a PEM layer and the catalyst backing layer. Hence, in actuality, an MEA may be composed of five layers: two catalyst backing, two electro-catalyst layers, and one PEM layer interposed between the two electro-catalyst layers. Most typically, the two electro-catalyst layers are coated onto the two opposing surfaces of a PEM layer to form a catalyst-coated membrane (CCM). The CCM is then pressed between a carbon paper layer (the anode backing layer) and another carbon paper layer (the cathode backing layer) to form an MEA. It may be noted that, some workers in the field of fuel cells refer a CCM as an MEA. Commonly used electro-catalysts include noble metals (e.g., Pt), rare-earth metals (e.g., Ru), and their alloys. Known processes for fabricating high performance MEAs involve painting, spraying, screen-printing and hot-bonding catalyst layers onto the electrolyte membrane and/or the catalyst backing layers.
An electro-catalyst is needed to induce the desired electrochemical reactions at the electrodes or, more precisely, at the electrode-electrolyte interfaces. The electro-catalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum supported on carbon. In real practice, an electro-catalyst can be incorporated at the electrode-electrolyte interfaces in PEM fuel cells by applying it in a layer on either an electrode substrate (e.g., a surface of a carbon paper-based backing layer) or a surface of the membrane electrolyte. In the former case, electro-catalyst particles are typically mixed with a liquid to form a slurry or ink, which is then applied to the electrode substrate. While the slurry preferably wets the substrate surface to some extent, it must not penetrate too deeply into the substrate, otherwise some of the catalyst will not be located at the desired membrane-electrode interface. In the latter case, electro-catalyst particles are coated onto the two primary surfaces of a membrane to form a catalyst-coated membrane (CCM).
Electro-catalyst sites must be accessible to the reactants (e.g., hydrogen on the anode side and oxygen on the cathode side), electrically connected to the current collectors, and ionically connected to the electrolyte membrane layer. Specifically, electrons and protons are typically generated at the anode electro-catalyst. The electrons generated must find a path (e.g., the backing layer and a current collector) through which they can be transported to an external electric circuit. The protons generated at the anode electro-catalyst must be quickly transferred to the electrolyte (PEM) through which they migrate to the cathode. Electro-catalyst sites are not productively utilized if the protons do not have a means for being quickly transported to the ion-conducting electrolyte. For this reason, coating the exterior surfaces of the electro-catalyst particles and/or electrode backing layer (carbon paper or fabric) with a thin layer of an ion-conductive ionomer has been used to increase the utilization of electro-catalyst exterior surface area and increase fuel cell performance by providing improved ion-conducting paths between the electro-catalyst surface sites and the electrolyte membrane. Such an ion-conductive ionomer is typically the same material used as the PEM in the fuel cell. An ionomer is an ion-conducting polymer, which can be of low, medium or high molecular weight. For the case of a PEM fuel cell, the conducting ion is the proton and the ionomer is a proton-conducting polymer. The ionomer can be incorporated in the catalyst ink or can be applied on the catalyst-coated substrate afterwards. This approach has been followed by several groups of researchers, as summarized in the following patents:    1) D. P. Wilkinson, et al., “Impregnation of micro-porous electro-catalyst particles for improving performance in an electrochemical fuel cell,” U.S. Pat. No. 6,074,773 (Jun. 13, 2000).    2) J. Zhang, et al., “Ionomer impregnation of electrode substrate for improved fuel cell performance,” U.S. Pat. No. 6,187,467 (Feb. 13, 2001).    3) I. D. Raistrick, “Electrode assembly for use in a solid polymer electrolyte fuel cell,” U.S. Pat. No. 4,876,115 (Oct. 24, 1989).    4) M. S. Wilson, “Membrane catalyst layer for fuel cells,” U.S. Pat. No. 5,211,984 (May 18, 1993).    5) J. M. Serpico, et al., “Gas diffusion electrode,” U.S. Pat. No. 5,677,074 (Oct. 14, 1997).    6) M. Watanabe, et al., “Gas diffusion electrode for electrochemical cell and process of preparing same,” U.S. Pat. No. 5,846,670 (Dec. 8, 1998).    7) T. Kawahara, “Electrode for fuel cell and method of manufacturing electrode for fuel cell,” U.S. Pat. No. 6,015,635 (Jan. 18, 2000).    8) S. Hitomi, “Solid polymer electrolyte-catalyst composite electrode, electrode for fuel cell, and process for producing these electrodes,” U.S. Pat. No. 6,344,291 (Feb. 5, 2002).    9) S. Hitomi, et al. “Composite catalyst for solid polymer electrolyte-type fuel cell and process for producing the same,” U.S. Pat. No. 6,492,295 (Dec. 10, 2002).
However, this prior-art approach of ionomer impregnation into the electrode layer and/or onto electro-catalyst particle surfaces has a serious drawback in that the ionomer, although ion-conducting (proton-conducting), is not electronically conducting. This is due to the consideration that a proton-exchange membrane, when serving as the solid electrolyte layer, cannot be an electronic conductor; otherwise, there would be internal short-circuiting, resulting in fuel cell failure and possible fire hazard. Such an electronically non-conductive material, when coated onto the surface of a catalyst particle or carbon paper fiber, will render the catalyst particle or carbon fiber surface electronically non-conductive. This would prevent the electrons generated at the catalyst sites from being quickly collected by the anode electrode substrate layer and the current collector, thereby significantly increasing the Ohmic resistance and reducing the fuel cell performance.
A measure of the fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. More effective utilization of the electro-catalyst, particularly through optimizing the electron and ion transfer rates, enables the same amount of electro-catalyst to induce a higher rate of electrochemical conversion in a fuel cell resulting in improved performance. This was the main object of the present invention.